1. Field of the Invention
The present invention relates to radio frequency communications devices. More particularly, it relates to allowing satellite transponders and ground terminals that utilize one polarization method (linear or circular polarization) to be able to cross-communicate with each other via wavefront multiplexing techniques. This offers many potential advantages, including but not limited to improved flexibility and increased efficiency of existing assets.
2. Description of Related Art
Satellite Communications (SATCOM) technologies have increased dramatically and have transitioned to IP-based services consistent with concepts for net-centric operations. Their increased use has resulted in a proliferation of IP-based products using satellites for back-bone or transport applications. On the other hand, high-speed satellite communications for access typically emanate from reflector antennas that basically radiate and receive in narrow beams.
Compatible polarization configurations between terminals and space assets are essential to efficient satcom links. It is generally true that the LP polarized terminals will use LP transponders, and CP terminals relay data via CP transponders. When ground terminals switch services from a provider with CP satellites to another one with LP satellites, their antenna polarizations are reconfigured accordingly, to prevent 3 dB SNR losses in receiving (Rx) functions on the one hand, and to avoid generating unwanted radiations in transmit (Tx) functions on the other hand. Our approach is very different than those of polarization switching, and would not require users to switch polarizations based on their equipment. Circularly Polarized (CP) users can use their existing terminals to relay data to CP destinations via linearly polarized (LP) transponders. There are no space asset losses due to the incompatibility. The method of LP space asset reorganization is the key for operationally circumventing the “incompatibility” issue.
A concept of the virtual-link concurrently utilizes N communications links organized by Wavefront (WF) Multiplexing (Muxing). A WF carrying a signal stream features a fixed spatial phase distribution among selected N parallel links, which support up to N orthogonal WFs carrying N independent signals concurrently from a source to a destination. The virtual link techniques are referred to as Orthogonal Wave-Front Diversity Multiplex (OWFDM), and the enabling signal structures as OWFDM waveforms.
Virtual links can be applied for satellite communications transporting data within a field of views common to selected transponders. Our proposed “Polarization Utility Waveforms” can successfully deliver signals via LP transponding satellites using CP ground terminals, and vice versa. They are engineered via techniques of signals spreading over multiple transponders. The waveforms may look like OFDM waveforms and also may appear as MIMO formats, but they are not. They are subsets of OWFDM waveforms and may feature unique format interconnecting OFDM and MIMO through an orthogonal signal structure.
WF muxing/demuxing techniques are powerful tools for path length equalizations among parallel paths/channels. SDS has applied these techniques for various applications; (1) Power combining from multiple transponders from the same satellites and/or different transponding satellites [1], (2) back channel equalization for ground based beam forming process in satellite applications [2], and (3) Distributed data storage [3].
Uniqueness of the OWFDM
Unlike OFDM for commercial wireless communications feature waveforms with multiple orthogonal sub-carriers uniformly distributed in a frequency band, our proposed OWFDM techniques will spread transmitting (Tx) signals into multiple channels with a unique phase distribution pattern, referred to as a wavefront (WF). These channels may be assigned to different frequency slots, time slices, polarizations, and/or orbital positions when these space assets are available. The selected multi-dimensional waveforms may be dynamic, and reconfigurable. There will always be embedded pilot signal streams through the same propagating paths, but distributed in phase distribution patterns orthogonal to the one which carries the desired signal stream. In short, the WFs are orthogonal to one another.
In general, the OWFDM waveforms must meet existing SATCOM polarization and frequency convention restrictions. At a ground station, transmitting (Tx) signals may be preprocessed by a WF multiplexer (muxer), which spreads coherent signals into multiple channels concurrently in the form of an orthogonal structure in a selected N-dimensional domain. The generated orthogonality is among multiple wavefronts (WFs). With N parallel propagating channels, there are N-orthogonal WFs available. Probing signal streams will be attached to at least one of them. The remaining WFs are available for various Tx signal streams.
Signals originated from a ground terminal propagating through various uplink carriers/paths, including multiple transponders on a satellite or among many satellites, and different down link frequencies/paths arriving at a destination feature differential phase delays, Doppler drifts, and amplitude amplifications/attenuations.
Post processing implemented at receiving (Rx) sites will equalize the differential phase delays, frequency drifts and amplitude attenuations among signals through propagating paths. Calibrations and equalizations take advantages of embedded probe signals and iterative optimization loops. There are no feedbacks required through back channels. As a result of the equalizations, the Rx WFs become orthogonal, and the attached signals streams are then precisely reconstituted by the WF demuxer.